Recently, diverse researches on providing multiple multimedia services including a voice service and transmitting high speed data while maintaining high quality in a radio communication environment are being conducted. In particular, technique for a multi-input multi-output (MIMO) communication system using a plurality of channels in a spatial area is being rapidly developed.
First, a general MIMO technique will now be described.
In brief, a MIMO refers to a method for improving data transmission and reception efficiency by using multiple transmission antennas and multiple reception antennas, shedding a system in which a single transmission antenna and a single reception antenna are used. Namely, the MIMO refers to a technique of seeking an increase in the capacity of data transmission or improvement of performance by using multiple antennas at a transmission end or at a reception end of a wireless communication system.
A multi-input multi-output technique (or a multi-antenna technique) is an application of a technique of collecting data fragments received by multiple antennas for a completion, rather than relying on a single antenna path for receiving a single entire message. Accordingly, a data transmission rate can be improved or an application range of the system can be increased over a particular data transmission rate.
A multi-antenna technique system using a MIMO scheme includes an open loop multi-antenna system that does not use feedback information from a reception end and a closed loop multi-antenna system that uses feedback information from the reception end. In detail, in the closed loop multi-antenna system, a reception end transmits feedback information regarding a channel status to a transmission end so that the transmission end can recognize the channel status based on the feedback information, thus improving performance of the multi-antenna system.
The closed-loop multi-antenna system uses an antenna grouping scheme in which a plurality of antennas of the transmission end are grouped, some of which are used for spatial multiplexing in order to improve a data transmission speed, while the others are used for diversity in order to improve reliability of a data transmission, an antenna selecting scheme in which the transmission end determines which one of antennas is optimal based on the channel information fed back from the reception end and dynamically selects an antenna involved in the MIMO, a precoding scheme in which the transmission end processes transmission data by using feedback information regarding a channel environment which has been from the reception end to minimize the influence of a channel, and the like.
In the method of transmitting the feedback, in general, channel status information (CSI) is fed back, and the channel status information feedback method may be divided into implicit CSI feedback information and explicit CSI feedback information. The implicit CSI feedback information is a method of changing or processing a channel status by using a method agreed by a mobile station and a base station, and such CSI information may include a CQI (Channel Quality Indicator), a PMI (Precoding Matrix Index), an RI (Rank Indicator), and the like. The explicit CSI feedback information is a method of transmitting channel status information itself or transmitting the channel status information maintaining its attribute, and in general, a channel vector/matrix, a channel covariance vector/matrix, etc are transmitted. The explicit CSI feedback information may be divided into full feedback (or analogue feedback) and a partial feedback depending on whether or not the entirety or a portion of the CSI is transmitted.
The explicit CSI feedback information includes a quantization method with respect to a long-term average of a spatial channel covariance. It is to divide an average with respect to channel covariance into diagonal components and off-diagonal components and perform quantization thereon as determined, thus reducing feedback overhead. For example, 1 bit quantization is performed on the diagonal components and 4 bit quantization is performed on the off-diagonal components.
As mentioned above, a method for minimizing feedback overhead is required in order to achieve an effective multi-input multi-output communication system.
However, the foregoing quantized CSI feedback method discussed in the related art is advantageous in that feedback overhead can be reduced independently from antenna configuration but has shortcomings in that the amount of feedback rapidly increases as the number of transmission antennas is increased. For example, 128-bit quantization is performed on eight transmission antennas, and such a large amount of feedback degrades the efficiency of the overall system.